Ethylene glycol and propylene glycol are valuable materials with a multitude of commercial applications, e.g. as heat transfer media, antifreeze, and precursors to polymers, such as PET. Ethylene and propylene glycols are typically made on an industrial scale by hydrolysis of the corresponding alkylene oxides, which are the oxidation products of ethylene and propylene, produced from fossil fuels.
In recent years, increased efforts have focussed on producing chemicals, including glycols, from renewable feedstocks, such as sugar-based materials. The conversion of sugars to glycols can be seen as an efficient use of the starting materials with the oxygen atoms remaining intact in the desired product.
Current methods for the conversion of saccharides to sugars revolve around a hydrogenation/hydrogenolysis process as described in Angew. Chem. Int. Ed. 2008, 47, 8510-8513.
An important aim in this area is the provision of a process that is high yielding in desirable products, such as ethylene glycol and propylene glycol, and that can be carried out in a commercially viable manner. A preferred methodology for a commercial scale process would be to use continuous flow technology, wherein feed is continuously provided to a reactor and product is continuously removed therefrom. By maintaining the flow of feed and the removal of product at the same levels, the reactor content remains at a more or less constant volume.
Continuous flow processes for the production of glycols from saccharide feedstock have been described in US 2011/0313212, CN 102675045A, CN 102643165A, WO 2013/015955 and CN 103731258A. A process for the co-production of bio-fuels and glycols is described in WO 2012/174087.
Continuous flow processes may be carried out in a reactor operating in essentially a plug flow manner. In such a system there is little or no back-mixing. At the start of the reactor there will be a high concentration of reactants. The concentration of starting materials decreases as the material moves through the reactor as a ‘plug’ and the reaction proceeds. Problems occur when the high concentration of reactants causes decomposition and the formation of by-products, leading to reduced yields of the desired products.
A continuous flow process with a high degree of back mixing may be also carried out, for example, in a continuous flow stirred tank reactor. In such a system the concentration of reactants at any one point will be much reduced, preventing any decomposition due to high concentrations. However, in such a process, as some of the reaction mixture is continuously removed from the reactor, there will be some material that does not react to completion. This results in a product stream that contains starting material and/or intermediates, reducing the overall yield of the process and requiring separation of the starting material/intermediate from the desired product and disposal or recycling thereof.
It would, therefore, be advantageous to provide a continuous process for the preparation of ethylene glycol and 1,2-propylene glycol from saccharide containing feedstocks in which substantially full conversion of the starting material and/or intermediates is achieved and in which the formation of by-products is reduced.